Retroreflection is a phenomena in which incident light is returned substantially back toward its source. This phenomena is extremely useful for providing signage and pavement markings which remain conspicuous at night with only vehicular illumination. Retroreflection is a cost efficient alternative to electrically operated signs and markings in many locations. Retroreflective signage (e.g., traffic signs, architectural locations, etc.) and pavement markings may be provided by installation of well known factory-prepared sheets and strips or, alternatively, by on-site application of paint-like liquids. Retroreflective signage and pavement markings are used in a variety of colors, depending in part upon the application. For instance, lane markings on pavement are typically yellow and/or white.
One early technique for imparting retroreflective effect to surfaces entailed application of a paint-like liquid to a roadway surface followed by sprinkling of glass beads or microspheres onto the wet liquid. The microspheres become partially embedded in the tacky liquid and served as retroreflective elements when the paint subsequently dried. U.S. Pat. No. 2,574,97 (Heltzer) discloses premixing of glass microspheres in a paint-like liquid prior to application to a roadway surface. One shortcoming of the resultant coatings is that the microspheres remain fully embedded in the resultant coating during and after drying. For this reason the coating is not immediately retroreflective, and becomes retroreflective only after traffic wears away some of the solidified paint so as to expose the front surfaces of the microspheres, thereby allowing them to function as retroreflective elements.
U.S. Pat. No. 3,251,704 (Nellessen) discloses a premixed retroreflective paint which is retroreflective immediately upon drying. The retroreflective paints disclosed therein employ spheroidal clusters of powdered pigment particles held together by a transparent binder. After application of the composition to a substrate, the coating dries and the clusters slide off the tops or front surfaces of the microspheres. Due to this advantageous property, the premixed microspheres at the surface are left unobscured by pigment, immediately able to function as retroreflective elements. It should be noted that typically a thin layer of binder resin, preferably substantially transparent, remains over the front surfaces of the microspheres, conforming to the surface thereof.
Although the premixed retroreflective paints disclosed in U.S. Pat. No. 3,251,704 offer the desirable qualities of single step application and immediate retroreflectivity upon drying, they are plagued by relatively high costs. The high cost is due in part to relatively complex and costly procedures of acid neutralization and firing at raised temperatures (see column 4, line 73, through column 5, line 3) required to produce a weatherproof embodiment of the clusters of pigment powder disclosed therein. Accordingly, it is desirable to either develop a less expensive colorant cluster possessing the capability of sliding off the front surfaces of microspheres as disclosed in U.S. Pat. No. 3,251,704 or, in the alternative, develop a less expensive process for production of such colorant clusters